Good morning: I use FirEfox to view PDF files. I have almost 6 or 8 PDF opened files, normally i use to read required data to my work. I dont know why, today the PDF files appears with black background and letters on black too. These documents has white background because i read it previously. I didnt find any option or theme that fix that. I let one as example: https://www.mouser.es/datasheet/2/1628/msd_4_a-3510387.pdf